This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. In this project, we will use the Geant4 nuclear-science package to simulate the effects of various collimator design parameters on sensitivity and resolution for single photon emission computed tomography (SPECT). We have previously developed analytic formulas that describe the geometric sensitivity and resolution (i.e., without photon penetration and scatter). We have developed preliminary models for penetration that need to be validated using computer simulation. We do not have at this time models for scatter. Thus, this project will utilize Geant4 to test models of penetration and scatter to determine their impact on clinical emission computed tomography with this new collimator. A secondary goal of this project is to develop algorithms for improved reconstruction quality, particularly with regard to 4D reconstruction (i.e., gated cardiac reconstruction).